body{
    margin: 0;
    padding: 0;
}
#box{
    width: 100px;
    height: 100px;
    position: absolute;
    top: 100px;
    left: 100px;
    background-color: red;
}
#aaa{
    width: 300px;
    height: 500px;
    background-color: #ccc;
}